The present invention pertains generally to dispensers used to produce on-demand foam-in-place packaging cushions and, more particularly, to an improved tip for such dispensers.
Foam-in-place packaging is a highly useful technique for on-demand protection of packaged objects. In its most basic form, foam-in-place packaging comprises injecting foamable compositions from a dispenser into a container that holds an object to be cushioned.
Typically, the object is wrapped in plastic to keep it from direct contact with the rising (expanding) foam. As the foam rises, it expands into the remaining space between the object and its container (e.g. a corrugated board box) thus forming a custom cushion for the object.
A common foamable composition is formed by mixing an isocyanate compound with a hydroxyl-containing material, such as a polyol (i.e., a compound that contains multiple hydroxyl groups), typically in the presence of water and a catalyst. The isocyanate and polyol precursors react to form polyurethane. At the same time, the water reacts with the isocyanate compound to produce carbon dioxide. The carbon dioxide causes the polyurethane to expand into a foamed cellular structure, i.e., a polyurethane foam, which serves to protect the packaged object.
In other types of foam-in-place packaging, an automated device produces flexible containers, e.g., in the form of bags, from flexible, plastic film and dispenses a foamable composition into the containers as the containers are being formed. As the composition expands into a foam within the container, the container is sealed shut and typically dropped into a box or carton holding the object to be cushioned. The rising foam again tends to expand into the available space, but does so inside the container. Because the containers are formed of flexible plastic, they form individual custom foam cushions around the packaged objects. Exemplary devices for automatically producing foam-in-place cushions in this manner are assigned to the assignee of the present invention, and are illustrated, for example, in U.S. Pat. Nos. 4,800,708, 4,854,109, 5,376,219, and 6,003,288, the entire contents of each of which are incorporated herein by reference.
One difficulty with the foamable compositions used to make polyurethane foam for foam-in-place packaging is that the foam precursors and resultant foam tend to have somewhat adhesive properties. As a result, the foamable composition tends to stick to objects and then harden thereon into foam. This tendency is particularly problematic at the discharge end of the dispenser, in which the foam precursors are mixed and from which they are ejected. As is known, the polyol and isocyanate foam precursors must be withheld from mixing with one another until just prior to injection into a bag or other container. In the most common type of dispenser, the two foam precursors enter the dispenser, mix with one another in an internal chamber in the discharge end of the dispenser to form a foamable composition, and then the resultant foamable composition exits the dispenser via a tip (i.e., a nozzle) mounted at the discharge end of the dispenser. As the dispenser operates over and over again, particularly in automated or successive fashion, foamable composition tends to remain and build up on the tip, harden into foam, and block the proper exiting of further foamable composition. When this occurs, the hardened foam either completely occludes the tip or causes the foamable composition to be diverted in random, uncontrolled directions, i.e., not the intended direction, and generally requires clean-up of any misdirected foam and a hasty shut-down of the dispenser.
A number of different techniques and devices have been developed to provide for the removal of the foam residue at the end of each dispense cycle, generally employing the use of a cleaning solvent that is capable of dissolving and thereby removing the foamable mixture from the tip and other parts of the dispenser. One such technique utilizes a porous discharge tip through which a mixture of cleaning solvent and air is pumped. The agitation of the solvent and air as they are mixed and passed through the porous tip produces a cleaning action that serves to rinse the foam and foamable composition from the tip. See, e.g., U.S. Pat. No. 4,898,327, the entire disclosure of which is hereby incorporated herein by reference.
An improvement on the foregoing technique employs a frusto-conical surface on the face of the tip to provide a greater linear area over which the cleaning action takes place. See, e.g., U.S. Pat. Nos. 5,255,847 and 5,950,875, the entire disclosures of which are hereby incorporated herein by reference.
A further improvement, which is also disclosed in the foregoing patents, is the use of a pressure transducer or other means to measure the pressure required to force the air and solvent through the porous tip. The air/solvent mixture generally removes most but not all of the foamable composition from the tip such that foam residue gradually accumulates on the tip. As the accumulation of the foam residue increases, so does the pressure required to force the air and solvent through the porous element. When the pressure reaches a predetermined limit, the operator is alerted to service the injection nozzle and, optionally, the dispensing system may be caused to shut down.
While the foregoing techniques have proven to be effective in providing dispensers with a longer service life, a need for further improvement remains. Specifically, the inventor has discovered that, during the dispenser cleaning cycle, the solvent migrates away from the porous tip and drips from a local low point on the discharge end of the dispenser (in use, the dispenser normally assumes a substantially vertical orientation, with the discharge end of the dispenser, at which the tip is located, pointed downwards). The solvent that drips from the dispenser is saturated with the mixed foam precursor chemicals and, as the saturated solvent drips off the discharge end of the dispenser, a small amount of mixed foam precursor chemicals are left behind as a residue, which cures into a solid. With each dispense and cleaning cycle the solid residue accumulates and grows, much like a stalactite in a limestone cave. Because the saturated solvent migrates away from the porous tip, the foam residue does not accumulate on the tip and, therefore, the pressure required to force air and solvent through it does not increase even as the size of the foam residue ‘stalactite’ does increase. Eventually, the foam residue grows to a size where it interferes with the proper operation of the foam dispensing system but without alerting the operator (due the absence of a pressure increase). Further, because the residue accumulates away from the porous tip, the cleaning action of the air/solvent mixture on the surface of the porous tip is ineffective in preventing or slowing the growth of the residue, thereby resulting in a shorter service life than would otherwise be desired.
Accordingly, a need exists to further extend the service life of foam-in-place dispensers and to improve the ability of foam-in-place systems to notify the operator before enough foam residue accumulates near the dispenser tip to inhibit the next dispense cycle or divert the foam stream in an undesired fashion.